Oximetry is the measurement of the oxygen status of blood. Early detection of low blood oxygen is critical in the medical field, for example in critical care and surgical applications, because an insufficient supply of oxygen can result in brain damage and death in a matter of minutes. Pulse oximetry is a widely accepted noninvasive procedure for measuring the oxygen saturation level of arterial blood, an indicator of oxygen supply. A pulse oximetry system consists of a sensor attached to a patient, a monitor, and a cable connecting the sensor and monitor.
Conventionally, a pulse oximetry sensor has both red and infrared LED emitters and a photodiode detector. The sensor is typically attached to an adult patient's finger or an infant patient's foot. For a finger, the sensor is configured so that the emitters project light through the fingernail and into the blood vessels and capillaries underneath. The photodiode is positioned at the fingertip opposite the fingernail so as to detect the LED emitted light as it emerges from the finger tissues.
The pulse oximetry monitor determines oxygen saturation by computing the differential absorption by arterial blood of the two wavelengths emitted by the sensor. The monitor alternately activates the sensor LED emitters and reads the resulting current generated by the photodiode detector. This current is proportional to the intensity of the detected light. The monitor calculates a ratio of detected red and infrared intensities, and an arterial oxygen saturation value is empirically determined based on the ratio obtained. The monitor contains circuitry for controlling the sensor, processing sensor signals and displaying a patient's oxygen saturation, heart rate and plethysmographic waveform. A pulse oximetry monitor is described in U.S. Pat. No. 5,632,272 assigned to the assignee of the present invention.
The patient cable provides conductors between a first connector at one end, which mates to the sensor, and a second connector at the other end, which mates to the monitor. The conductors relay the emitter drive currents from the monitor to the sensor emitters and the photodiode detector signals from the sensor to the monitor.
The patient cable conductors may also relay information to the monitor regarding sensor status. For example, FIG. 1 shows a prior art pulse oximetry system 100 that detects whether a sensor 110 is connected to a monitor 170, either directly or through a patient cable 140. The sensor has a conductor pair 120 (shown dashed) that corresponds to pinouts on a sensor connector 130. The monitor 170 also has a conductor pair 180 (shown dashed) that corresponds to pinouts on a monitor connector 190. The patient cable 140 mates with the sensor 110 at one end and the monitor 170 at the other end, so that the sensor conductor pair 120 becomes electrically connected to the monitor conductor pair 180. A short-circuit conductor 122 connects the sensor conductor pair 120 together at the sensor 110. An open circuit detector 172 within the monitor 170 senses the conductance across the monitor conductor pair 180. When the sensor 110 is plugged into the patient cable 140, the sensor conductor pair 120 is connected to the monitor conductor pair 180, and the conductance measured by the open-circuit detector indicates the presence of the short-circuit conductor 122. When the sensor 110 is unplugged from the patient cable 140, the sensor conductor pair 120 is disconnected from the monitor conductor pair 180, and the conductance measured by the open-circuit detector 172 indicates an open-circuit. Hence, the combination of the short-circuit conductor 122 and the monitor open-circuit detector 172 functions to detect a no-sensor condition. This is a useful indicator for the monitor signal processor, which can distinguish between a sensor malfunction and a no-sensor condition, providing a display to the user accordingly.